Local police say the gunmen raided two villages overnight.
Somalia’s al-Shabab group said it had carried out the attack, telling Reuters that its “operations in Kenya will continue”.
At least 48 people died after the al-Qaeda-linked group attacked hotels and a police station in the town on Sunday.
Al-Shabab said it was revenge for the presence of Kenyan troops in Somalia and the killing of Muslims.
Kenya sent troops to Somalia in 2011 to help the weak UN-backed government defeat the militants.
“We raided villages around Mpeketoni again last night,” al-Shabab spokesman Sheikh Abdiasis Abu Musab told Reuters on Tuesday.
He was later quoted by the AFP as saying that the militants “have been going to several places looking for military personnel”. He said most of those killed were police officers and wildlife wardens.
Kenyan police said that the gunmen raided two villages in the Poromoko district near Mpeketoni.
The authorities said the militants jammed a telecommunication system before the killings to prevent villagers from sounding the alarm.
In Sunday’s attack, al-Shabab appeared to target men, in many cases leaving their wives and children unharmed.
Mpeketoni, near Lamu Island, is not a tourist resort. It appears the attackers were not interested in foreigners or their interests, our correspondent says.
After last year’s Westgate attack in Nairobi – al-Shabab’s most deadly raid in Kenya – the group received a lot of criticism for killing women and children.
Kenya has been on high alert recently following warnings that al-Shabab was planning more attacks.
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